Quad-band bandpass filter design by embedding dual-band bandpass filter with dual-mode notch elements

A quad-band bandpass filter of compact dimensions by embedding several dual-mode notch elements into a stub-type dual-band bandpass filter is presented, and the four passbands are achieved by splitting the two passbands of the dual-band bandpass filter with two notch bands generated by the dual-mode notch elements. A prototype filter with passbands located at 1.4-1.7, 2.1-2.3, 3.3-3.6 and 5.2-5.4 GHz is designed with the help of optimisation methods. Good agreement between simulations and measurements are achieved, demonstrating good features such as controllable and wide bandwidths, higher filter order, compact dimensions and high isolations.